Telephone system



Jan, 26, 1932.. T. F. cRocKER TELEPHONE SYSTEM Filed Feb. 26, 1951 2 Sheets-Sheet l WW W M E Jan. 26, 1932.. CRQCKER 1,842,659

TELEPHONE SYSTEM Filed Feb. 26, 1931 Z Sheets-Sheet 2 Ifi'EEFTEF Thumas" F: Cracker Patented Jan. 28, 1932 il iit 'l ENT OFFEQFI THOMAS E. CROGKER, OF CHICAGO, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS, TO

ASSOCIATE-13' ELECTRIC LABORATORIES, INC.,

TION, 0F DELAVARE OF CHICAGO, ILLINOIE, .A. CORPORA- TELEPHCNE SYSTEM Application filed February 26, 1931.

This invention relates, in general, to branch ez-zchange telephone systems and more particularly to private automatic branch exchange telephone systems in which local calls are completed by means of subscriber-control autranatic switches, and in which incoming calls are intercepted and completed by an attendant. The invention is concerned primarily with the attendants equipment and with the trunking arrangements for connecting a branch exchange with an exchange oi? the public telephone system.

fflystems of the above type may be arranged with the subscribers lines accessible via antomatic switches controlled by the attendant, or the subscribers lines may be multipled manual jac (s at the attendants switchboard, in which case incoming calls are to be completed directly by means oi? cord circuits. The present invention contemplates the latter arrangement and one of the main features is the provision of a new and improved cord circuit for completing incoming calls. The cord circuit is arranged so that a. bran::lrcxchange subscribers line can be Freed from an incoming connection and .uiadc available for other calls without waiting tor the attendant to remove the plug of the cord from the line-multiple jack. The cord is iurther provided with a recall signal, operable under control of the branchcnchange subscriber.

The trunk, which comprises another feature oi? the lllVtlltlOll. is of the same general type as the trunk disclosed in the Lomax Patent No. 'ififilflfl ih, of December 10, 1929. The trunk, however, is adapted for connecting a branch exchange with a manual main ill exchange and is arranged to prevent the acl0 rudcntal release oi the branclrcxchange switches when an outgoing call over the trunk is answered 2 t the main exchange. This is attained by tu'oviding the lnanch-exchange repeater with a holdi g circuit which functions until the answe 1g plug has been fully and cerrcclv positioned in the answering jack, v-rheleupon the holdin circuit is transl'rrrr-d to the answering end of the trunk.

"i he vari u ieaturcs of the invention, in- 59 eluding those specifically referred to and Serial No. 518,429.

others, together with the operation of a telephone system embodying the same, will be described in detail hereinafter, reference be ing had to the accompanying drawings, comprising l and 2. Fig. 1 shows a branchneetors.

- Lineswitch LS is of the well-known rotary type, although any suitable non-numerical lincswitch or line finder may be employed if desired. Lineswitch LS, together with other similar lineswitches, has access to local first selectors, such as selector 3, shown only in skeleton form. Selector S and similar selectors have access to the usual local trunks terminating inbranch-cxchange switches (not shown) and in one level, for example, the tenth level, access is had to trunks extending to the main exchange.

Cord circuit C, also shown in Fig. 1 is designed for connecting the interofiice trunks with branch-exchange subscribers lines. I The cord is provided with an. answering plug Pl for connecting with the trunk jacks and a calling plug P for connecting with the linemultiple jacks. The usual supervision is provided by the lamps L and L1 and key K pro vides for connecting up the operators set and initiating the automatic ringing.

Referring now to Fig. 2, the trunk line T is one oi. a group of tri'lnks connecting the branch exchange with a manual main eX- change. The trunk is accessible in the banks of the branch exchange selectors, such as selector S, and also terminates in jacks at the attendants switchboard. Jack J2 is used for answering calls and jack J1 is employed for night service, which, in this case, is supplied by connecting the trunk with a particular subscribcrs line when the attendant leaves lineswitch LS will operate and select an idle first selector. such as the selector-S. The

subscriber will then dial the number of'the wanted subscriber operating selector S and other branch-exchange switches which will complete the connection, in the usual way. Ground is returned by the branch-exchange connector over the release trunk to hold up the lineswitch LS, and this ground, by way of conductor 3, marks the line busy in the banks ofthe connectors and also at the attendants switchboard.

A call to the main exchange is made by dialling. the exchange digit, which, for convenience, may be assumed to the digit 0, whereupon selector S is operated to a level in which the trunk lines T are accessible and an idle trunk is selected. The line relay at the manual exchange is operated over one trunk conductor and this relay lights the line lamp of the selected trunk. The manual operator answers by means of the usual cord circuit and further extends the call as required. A holding ground is returned for the lineswitch by the trunk relay group. This holding ground is supplied by a relay in series with the trunk conductor. The relay is energized in a local circuit until such time as the call is answered, whereupon the circuit for the relay is transferred to the battery-feed bridge of the answering cord circuit.

In order to call a branch-exchange subscriber, the main-exchange operator will select an idle trunk line, such as the trunk line T. and insert the calling plug of the cord into the jack J3. The trunk line will be'marked busy at the branch exchange immediately upon its being taken into use, but the attendant will, not be signalled at this time. When the main-exchange operator desires to signal the attendant, ringing current will be applied to the trunk and the line lamp L2 will light. The attendant will answer the call by inserting the answering plug P1 of the cord circuit, such as G, into the jack J2. Key K will be operated to connect up the attendants telephone set, and the main-exchange operator will then advise the attendent as to the particular line wanted. The attendant will. test the subscribers line for its busy or idle condition by touching the tip of the plug P to the sleeve of the line-multiple jack. If the line is busy, the attendant will receive the usual busy click and will inform the calling operator of this condition. If the line is idle, the attendant will insert the plug P into the line-multiple jack and momentarily operate key K to ringing position. The branch-exchange subscriber will be rung intermittently until the call is answered, whereupon the ringing current will be disconnected and a talking circuit will be completed between the subscriber and the mainexc'hange operator. In case the branch-exchange subscriber desires to recall the attendant for any reason, the subscriber will actuate his calling device, whereupon the supervisory lamp L ,fiashes: to attract the attendants notice. When the conversation is finished, the branch-exchange subscriber will hang up and the line will immediately be freed from the connection. The plug of the cord C may remain in the multiple jack without busying the line, and therefore, either outgoingor incoming calls may be completed.

Having briefly described the invention, the operation of a system embodying the same will now be described in detail. It will first be assumed that the subscriber at substation A is calling a main-exchange subscriber. When the receiver is lifted at the substation'A, a loop circuit is closed for line relay 5 and the relay operates and completes the testing circuit of the switch. If the trunk on which the switch wipers are standing is busy, magnet 6 will energize and step the wipers to the next set of contacts. hen an idle trunk is found, for example, the trunk comprising conductors 121 i terminating in selector S, switching relay 1 will operate and extend the calling loop throughto the line relay of the selector by way of contacts 7 and 8. The selector will return a holding ground over release-trunk conductor 13 in the usual way to hold line switch LS in operated position after relay 5 restores. The subscriber then dials the digit assigned for main-office calls, e. g., the digit 0, and wipers 1517 of selector S are'raised to the level in which the main-office trunk lines are accessible. The selector rotates and tests for an idle trunk in the usual way, and when one is found, switches through and extends the calling line loop to the trunk repeaters It will be assumed. for convenience, that the trunlrline selected is the one comprising conductors 18 20, terminating in the trunk apparatus shown in Fig. 2. The calling loop is extended over conductors 18 and 20, the former which may be traced through the winding of relay 101, trunk conductor 131, contacts 135, upper winding of line relay 133 to ground. Conductor 20 extends by way of contacts 120 and winding of relay 102 to battery. Relays 101, 102, and 133 operate over the above circuit. Relay 133 completes acircuit at contacts 134 for the line lamp L3 the main exchange. and this lamp glows to notify the main-exchange operator of the incoming call. Relay 102 operates and completes a circuit at contacts 114 for relay 103 and at contacts 115, completes a holdin circuit tor relay 101 so that this relay Will be maintained operated independent of the ground through the upper winding of relay 1533. Resistance 111 is of such value that relay 133 is not shunted out, but remains operated and relay 101 will also remain operated even though the contacts 135 may be opened. Relay 101 closes an additional circuit -tor relay 103 at contacts 118. Relay 103 operates, opens the bridge across the trunk which includes relays 105 and 100 at contacts 110, and at contacts 11]", applies ground to conductor 19 and the sleeve of the jack J2 to hold up the preceding switches involved in the connection, to mark the trunk busy at the attendants switchboard, and to operate the visual busy signal 107.

The operator at the main exchange on noting the ligl'ited condition of the lamp L3, inserts the answering plug oi a cord circuit into jack J8. Relay 132 operates over the sleeve oi the jack and plug, and at contacts 135 disconnects relay 133 from the trunk. The cord, as previously stated, is arranged to supply battery and ground to the calling subscriber and when the plug is inserted in the trunk, a circuit is completed from battery over the tip contacts of the plug and jack, conductor 130, contacts 122 and 116, winding of relay 104 to ground. Relay 104 operates, disconnects relay 102 from conductor 20. and extends this conductor by way oi contacts 121 to conductor 130 of the trunk. lteray 104 also applies an additional ground to the release-trunk conductor at contacts and completes a locking circuit for itself at contacts 123. Relay 102 releases, opens the original energizing circuit of relay 103 at contacts 114, and opens the temporary holding circuit for relay 101 at contacts 115. ltelay 101 now remains operated in series with the battery-feed relay of the cord circuit connected up at the main exchange. Relay 103 remains energized over the circuit including contacts 113, and relay 104 remains energized in series with resistance 112. The operator may now converse with the calling subscriber and further extend the call in the usual manner by means of the cord circuit.

At the termination of the conversation, the branch-exchange subscriber will restore his receiver, thereby opening the loop over which relay 101 maintained energized. Relay 101 will restore and open the circuit of relay 108 which will also restore. Belay 103 opens a point in the circuit of relay 104 at contacts 116. removes ground from the releasatrunlr conductor at contacts 117, and recoi'mects relays 105 and 100 across the trunk conductors at contacts 119. Relay 104 is slow to release and remains operated when its circuit is opened at contacts 116 by relay 103. Relay 103, as previously stated, completes the bridge across the trunk line and relay 106 operates in series with the battery feed relay in the cord circuit. Relay 106 is of relatively high resistance and does not permit the battery-feed relay to operate in series therewith so that the operator is given disconnect supervision. Relay 106 completes a new circuit for relay 104 at contacts 128, so that relay 104 remains operated and maintains the busying ground on release-trunk conductor 19 by way of contacts 125. The operator will note the lighted condition oi. the disconnect supervisory lamp and withdraw the plug of the cord circuit from the jack, whereupon relay 106 deenergizes and opens the circuit of relay 104. Relay 104 then denergizes and removes the ground from the release-trunk conductor, thereby permitting the preceding switches to release, and the trunk is rendered avail able for other calls.

A call from the main exchange to the subscriber A at the branch exchange will now be considered. 1 The main-exchange operator will select an idle trunk line, for example, the trunk line T, and will insert the calling plug of a cord circuit into the jack J3. Relay 106, which is bridged across the conductors of the trunk, operates in series with the battery-feed relay of the cord, and at contacts 128, completes a circuit for relay 104 in series with resistance 112. Relay 104, upon operating, prepares the talking circuit by clos ing contacts 121, prepares a locking circuit for relay 105 at contacts 124, and applies ground to release-trunk conductor 19 at contacts 125 to mark the trunk busy in the banks of the selector switches of the branch exchange. The calling operator now applies ringing current to the trunk and relay 105, whose upper winding is bridged across the trunk conductors in series with condenser 129, is operated. Relay 105 completes a locking circuit for its lower winding at contacts 127, and at contacts 126 disconnects its upper winding "from in bridge of the trunk. Lamp L2 is connected in parallel with the lower winding of relay 105 and is lighted from ground by way of contacts 118, 124, and 127. Upon noting the lighted condition of lamp L2, the branch-exchange attendant will insert the answering plug P1 of a cord circuit, such as cord G, into jack J2. Jack spring contacts 108 are closed to complete a circuit from battery by way of resistance 109 for relay 104. Relay 104 was previously onergized as described and the latter described circuit merely serves to hold this relay operated when its initial energizing circuit is opened. A circuit may now be traced from battery, winding of relay 80. of the cord, sleeve contact of the plug P1, sleeve of the jack J2 to ground at contacts 125. Relay 30 operates, closes the talking strands of the cord circuit at contacts 72 and 74, and prepares various holding circuits at contacts 73. Relay 29 is bridged across the talking strands of the cord and operates'in series with the battery-feed relay of the calling cord. Relay 101 of the trunk circuit operates in series with relay 29 and the battery feed relay of the calling cord, and at contacts 113 completes a circuit for relay 103. Relay 103 applies an additional ground to the release-trunk conductor at contacts 117, and at contacts 119 opens the bridge including relay 106, which is normally connected across the trunk line. Relay 106 restores and opens the circuit of relay 104 at contacts 128, but this is without effect since relay 104 is now energized in series with resistance 109. The operation of relay 29 of the attendants cord is without effect at this time. The attendant now actuates lrey' K to the right, thereby bridging her telephone sets across the talking strands of the cord at contacts 64 and 65. Atone circuit is prepared at contacts 62, the busy-test circuit is prepared at contacts 63, and a point in the locking circuit of certain relays is opened at contacts 66. The attendant may now converse with the calling operator and determine the particular branch-exchange line with which connection is desired. Assuming that the call is for the subscriber at substation A, the attendant will test the line for its busy or idle condition by touching the tip of the plug to the sleeve of jack J. If the line is engaged in another connection, there which indicates to her that the line is engaged in another connection. The'attendant will then notify the calling operator of the busy condition of the line and the connection will be released as will be described subsequently. Should the attendant disregard the busy indication and plug into the line jack, this wouldbe without effect as relay 22 would be shunted by the ground on the test conductor of the line.

In case the wanted line is idle when the connection is extended asabove described, the attendant will not receive the busy click on testing the line and will, therefore, insert the plug P into the multiple jack J. A circuit is thereby completed from ground, contacts 49, winding of relay 22, sleeves of the plug and ack, conductor 3, winding of cutoff relay 4, magnet 6, to battery. Relays 4 and 22 operate in this: circuit, the former removing a normal battery and ground connection from the line in the usual manner, and the latter preparing the ringing circuit at contacts 32 and preparing a holding circuit to relay 23 at contacts 33. The attendant now actuates key K to the left, thereby closing a circuitfrom ground, contacts 60, winding of relay 23 to battery. Relay 23 operates and completes a locking circuit for itself at contacts 35, completes the ringing circuit at contacts 34 and 39, and opens the supervisory circuit at contacts 37. Ringing current is now applied intermittently to the line over a circuit extending from grounded generator GEN, front contacts of the interrupter relay, contacts 34, 40, and 32, tip contacts of the plug and jack, conductor 1, through the bells and condenser of the substation, conductor 2, contacts 21, ring contacts of the jack and plug, contacts and 39, lower winding of relay 24 to battery. The bell at the called substation rings intermittently. Relay 24 is provided with the usual copper slug and sleeve so that it will not respond to the ringing current. scriber answers, a direct-current bridge is closed at the substation and relay 24 operates over the above-described circuit, or, in case the call was answered during a silent period, ground by way of the back contacts of the interrupter relay will be substituted for the grounded generator. Relay 24, on operating, completes a locking circuit for itself at contacts 42, opens the ringing circuit at contacts 40 and 45, and completes the talking circuit at contacts 41 and 44, and at contacts 43 closes an additional circuit for relay .29. The called subscribers circuit is now extendedthrough the upper and lower windings of relay 27 to battery and ground and relay 27 operates. A circuit is completed for relay 26 at contacts 59 and a tone circuit is completed at contacts 57. Relay 260perates and prepares a circuit forrelay 25 at contacts 52, prepares the supervisory circuit at contacts 54, and connects an additional ground to the circuitiof relay 22 at contacts 56. V

The attendant will normally restore key K as soon as the automatic ringing of the called subscriber has started. In case the attendant neglects to restore this key to normal she Wlll be given a tone s1gnal as soon When the sub I as the call is answered. The circuit for apthe subscriber will depress the switchhook momentarily or, preferably, dial the digit one. Relayf 27 will restore responsive to the open impulse and complete a circuit by way of contacts 58 and 52 for relay 25. ldelay 26 is slow to release and remains operated when its circuit is momentarily opened at conlia 59,. Belay 25 operates and coinpletesa locking circuit for itself at contacts as, completes a circuit for supervisory lamp ll at contacts 47, and opens the original. circuit of relay 29 at contacts 51. Relay 27 remains energized after the impulse is received and maintains the circuit for relay 26. Lamp L now flashes from the interrupted ground by way of interrupter INT. 1, contacts a? and 54-, lamp L to battery. On noting the flashing of the lamp, the attendant will again enter the connection by operating key K to the right. "he locking circuit of relay 25 is opened at contacts 66 and the relay restores and opens the circuit of the supervisory lamp at contacts 47. After conversing with the subscriber, the attendant will withdraw the calling plug from the jack J and insert it in the jack of the line to "which the call is to be transferred. The procedure followed in completing the new call is exactly the same as that described for completing the call to substation A.

The main exchange operator may also recall the attendant by applying ringing current to the trunk. This current operates relay 28 whose upper winding is bridged across the talking strands of the cord in series with condenser 80. Relay 28 completes a locking circuit for its lower Winding at contacts 68, opens its original energizing circuit at contacts 67, and connects lamp L1 to interrupter INT. 1 at contacts 69. Lamp L1 flashes to notify the attendant of the recall.

At the termination of a call such as that above-described, the branch-exchange subscriber will hangup his receiver. The opening of the line loop permits relay 27 to release and complete the previously traced circuit for relay 25. In this case relay 2? remains 'unoperated and, after a short delay, relay 26 releases. Relay 26 completes a circuit for supervisory lamp L1 at contacts 53 and removes ground from the sleeve relay circuit at contacts 56. Sleeve relay 22 of the cord and cut-oif relay 4. of the line-switch release, the latter restoring the line to normal condition so that it can be used for other calls. Relay 22 opens the talking strands of the cord at contacts 32 and opens the circuit of relay 23 at contacts Relay 23 restores, opens the ringing circuit at contacts 34: and 39, opens the locking circuit of relay 24L at contacts 36, closes the supervisory circuit at contacts 37, and completes a holding circuit for relay 29 at contacts 38. Relay 24 restores and opens the loop circuit over which relay 29 was held ope-rated during the connection. The latter relay is now energized in a circuit which may be traced from ground, resistance 81, contacts 38 and 50, winding of the relay, tip contact of theplug and jack, contacts 121, trunk conductor 130, ring contacts of the jack and plug at the main exchange, to battery through the battery feed relay of the cord. The battery feed relay does: not remain energized in series with relay 81 and on restoring, gives the usual disconnect su: pervision. In case the operator desires to tall: to the branch-exchange attendant, the plug will be left inthe jack and relay 29 will remain operated. Lamp L1 will flash as it is connected to interrupter INT. 1 via contacts 71, 53 and 4:7. Lamp. L will glow steadily as soon as relays 23 and 26. have restored and will notify the attendant that the branchezrchange subscriber has. hung up.

In case the main-exchange operator does not desire to recall the branch-exchange attendant, the connection will be pulled down at the main exchange as soon as the disconnect signal is received as above described. The circuit of relay 29 Will be opened and the re lay in turn will open the circuit of lamp. L1 at contacts 71. Lamp L will glow steadily as in the previous case to give the attendant disconnect supervision.

The attendant on receiving the disconnect signal will pull down the connection and re store the apparatus to normal condition. It may be noted, however, that the branch-exchange subscribers line is not prevented from calling or receiving calls in case the attends ant delays in pulling down the connection. The ground connections to the sleeve circuit will be removed by the release of relay 26 and the operation of relay 25 as previously described. The calling strands of thecord will be open at contacts 32 and 39 so that relay 27 cannot operate when a loop circuit is again closed at substation A. Further, when a call to or from substation A is established under the above conditions, the ground on the release trunk will prevent relay 22 of the cord circuit from operating in case the attendant should operate key K and release relay 25. The attendant is thus prevented from entering the established connection.

What is claimed is:

1. In a link circuit temporarily connected with a line, a signal, means responsive to a momentary interruption of the line for op erating said signal in a distinctive manner, and means responsive to a prolonged interruption of the line for operating said signal in a different manner and for freeing the line from the connection with the link.

2. In a telephone system, a line terminating in a jack, a cord circuit for extending a connection to said line via said jack, means in the cord for marking the line busy, a signal in the cord operated in a distinctive man lOG ner responsive to a momentary interruption of the line, and means in the cord for operating said signal in a different manner and for disabling said busy marking means responsive to a prolonged interruption of the line.

3, A telephone system in which connec tions are extended to called lines by means of a cord circuit and in which the cord circuit is provided with a supervisory signal operated in a distinctive manner. under control of the called subscriber, and with means also controlled by the called subscriber for operating the signal in a different manner and for freeing the line from the connection.

4. In a link circuit for connecting a calling 'line and a called line, means for applying ringing current to the called line, means controlled over the called line for disabling said first means, a supervisory signal, and means controlled over the called line after said first means has been disabled for operating said signal in .eitherof two distinct manners.

5. In a link circuit for connecting a calling line and a called line, means for placing said link in condition to automatically signal the called line, means for changing the link from signalling to talking condition responsive to the called subscriber answering, and a supervisory lamp rendered effective by said last means to flash responsive to a'momentary interruption of the called line and to glow steadily responsive to a prolonged interruption of the called line.

.6. In a link circuit for connecting calling and" called lines, two supervisory lamps, means responsive to a momentary interruption of the called line for causing one of said lamps to glow intermittently, and means responsive to the called subscriber hanging up before the calling subscriber for causing said one lamp to glowsteadily and the other of said lamps to glow intermittently.

7. In an operators link circuit temporarily connected with a line, means for preventing the line from originating or receiving calls, a signal, means controlled over the line for operating said signal in a distinctive manner to recall the operator, and means controlled over the line for operating said signal in a different manner to notify the operator of the termination of the call and for disabling said first means to enable the line to originate or receive calls prior to the disconnection of the link.

8. In an operators link circuit temporarily connected with a line, means for preventing the line from originating or receiving calls, a signal, means responsive to a momentary interruption of the line for operating said signal in a distinctive manner, and means responsive to a prolonged interruption of the line for operating said signal in a different manner and for disabling said first means to permit the line to originate or receive calls independent of the connected link.

9. In an operators link circuit temporarily connected with a line, means for preventing the line fromcalling or being called, a supervisory lamp, means responsive to a momentary interruption of the line for causing said lamp to flash at a predetermined rate, and means responsive to a prolonged interruption of the line for causing said lamp to glow steadily and for disabling said first means to permit the line to call or be called while the link circuit remains connected thereto. 10. In a telephone system, a subscribers line terminatingin a manual jack and in an automatic switch, a cut-oft relay for disconnecting the line from said switch, a cord circuit for extending connections to the line via said ack, means responsive to the establishmentof a'connection for completing an operating circuit for said relay, a signal in said cord circuit, means in the cord controlled over the line for causing said signal to operate intermittently, and means in the cord controlled over the line for causing said signal to operate continuously and for opening the circuit of saidrelay.

11. In a telephone systemfa subscribers line equipped with a calling device and terminating in a manual jack and an automatic switch, a cord circuit for extending connections to the line via said jack, busying means in the cord circuit for preventing the line fromoriginating or receiving calls 'while the cord is connected thereto, a signal in the cord, means in the cord responsive to the operation of the calling device of the connected line for causing said signal to operate intermittently, and means in the cord responsive to the subscriber on the connected line hanging up after havinganswered a call for causing said signal to operate continuously and for disabling said busying means.

12. In a telephone system, a trunk line comprising two conductors connecting a main exchange and a branch exchange, a relay at the main exchange connected to one of said conductors in series with the positive pole of a'battery, a relay at the branch exchange connected to the other of said conductors in series with the negative pole of a battery,

means responsive to the seizure of the trunk line at the branch exchange for connecting said two conductors to operate both said relays in series, and means responsive to the operation of saidsecond relay for connecting the positive pole of a battery to said one conductor of the trunk to maintain the relay operated independent of said first relay and battery. a

13. In a telephone system, atwo-conductor trunk line connecting a main exchange and a branch exchange, a relay at the branch exchange, a second relay at the main exchange, an initial series energizing circuit for said relays inclu'ding'a source of current ion Iii

at the branch exchange, a conductor of the trunk line and ground at the main exchange, and means responsive to the operation of said first relay for connecting ground through a resistance to said trunk conductor at the branch exchange to hold the said first relay operated without deenergizing said second reiay.

i l. In a. telephone system, a tWo conductor trunk line connecting a main exchange and a branch exchange, a relay at the branch eX- change, a second relay at the main exchange, means responsive to a call over the trunk line for completing an initial series operating circuit for said relays including a source of current at the branch exchange, one conductor of the trunk line and ground at the main exchange, and means responsive to the operation of said first relay for closing a parallel local circuit for maintaining the relay operated independent of said second relay While saio second relay is maintained operated over said initial circuit.

15. In a telephone system, a main exchange and a. branch exchange, a two-conductor trunk line connecting said exchanges and terniii'lating in switch banks at the branch we change and in a manual jack at the main exchange, a line relay for the trunk line at the n ain exchange, a call signal controlled by said relay, means for seizing the trunk line at said switch banks to extend a call to the main exchange, means responsive to said seizure for connecting a source of current to only one of the trunk conductors to operate said line relay and said signal, and means responsive to the answering of the call for disabling said relay and said signal, for disconnecting said source of current from said 40 one conductor and for connecting the calling line to the two conductors of said trunk line. In Witness whereof, I hereunto subscribe my name this twentieth day of February, it. D. 1931. THOMAS F. CROCKER. 

